In depth study of the compensation in annealed heavily carbon doped GaAs

Heavily C-doped GaAs epitaxial layers with holes concentrations ranging from 10¹⁹ to 1.6×10²⁰ cm⁻³ have been grown by metal organic vapour phase epitaxy (MOVPE) using CCl₄ as C-growth precursor. The carbon doping characteristics of GaAs epilayers have been investigated by optimizing the V/III ratio and the growth temperature. Additional informations have been extracted from the evolution of the in situ reflectivity signal during the growth of GaAs:C. The appearance of discernible oscillations in the reflectivity response indicates the high carbon incorporation and the good surface quality in spite of the CCl₄ etching effect. The hole concentration tends to saturate at about 1.5×10²⁰ cm⁻³. The comparison between Hall effect measurements realized on sets of as grown and annealed layers, and theoretical calculations of the mobility lead to the determination of the compensation ratio of the samples.The lattice matching conditions were systematically investigated by using high X-ray diffraction measurements from (004) and (115) planes. A comparison between the experimental mismatch and the one calculated with the Vegard's law allows the estimation of the possible origin of the compensation. Secondary ion mass spectrometry, scanning electron microscopy and atomic force microscopy have been used as complementary tools to characterize the films.     

Carbon doping and etching in GaxIn1−xAsyP1−y on GaAs substrates using CBr4 by metalorganic chemical vapor deposition

Carbon doping and etching by CBr₄ were studied for Ga_xIn_1−xAs_yP_1−y (0≤y≤1) on GaAs grown by metalorganic chemical vapor deposition. It was found that the hole concentration drastically decreases with decreasing y when the flow rate of CBr₄ is constant. When y is under 0.5, the conduction type of GaInAsP changes ton-type. In the region of 0<y<0.6, the surface morphology was degraded and the carrier compensation became higher than could be estimated from the C concentration. This seems to be due to the micro defects because this range of composition is within the unstable region which is theoretically predicted. The etching effect by CBr₄ was observed during the growth. The rate of etching for InAsP component is about three times larger than that for the GaAsP component. The thermodynamic analysis suggests that the etching is due to the increase of the partial pressure of GaBr and InBr.     

Carbon doped GaAs grown in low pressure-metalorganic vapor phase epitaxy using carbon tetrabromide

Carbon tetrabromide was used as carbon source for heavily p-doped GaAs in low pressure metalorganic vapor phase epitaxy (MOVPE). The efficiency of carbon incorporation was investigated at temperatures between 550 and 670°C, at V/III ratios from 1 to 50 and carbon tetrabromide partial pressures from 0.01 to 0.03 Pa. Hole concentrations from 8 × 10¹⁷ to 5 × 10¹⁹ cm⁻³ in as-grown layers were obtained. After annealing in nitrogen atmosphere at 450°C, a maximum hole concentration of 9 × 10¹⁹ cm⁻³ and a mobility of 87 cm²/Vs was found. At growth temperatures below 600°C, traces of bromine were detected in the layers. Photoluminescence mapping revealed an excellent doping homogeneity. Thus, CBr₄ is found to be a suitable carbon dopant source in MOVPE.     

Comparative study between electrical, optical and structural properties of annealed heavily carbon doped GaAs

Heavily C-doped GaAs epilayers have been grown by atmospheric pressure metalorganic vapor phase epitaxy with hole concentration ranging from 2×10¹⁹ to 1.6×10²⁰ cm⁻³. In order to study the stability of C acceptors over this range, the films have been annealed at 810 °C for 10 min under two mixture gas AsH₃+H₂ or only N₂. Annealing of the layers resulted in a decrease in carrier concentration, carrier mobility and lattice mismatch with undoped GaAs. The lattice matching conditions of C-doped GaAs layers were systematically investigated by using X-ray high-resolution diffraction space mapping. The comparison between electrical and structural before and after annealing of layers properties indicates that the simultaneous decrease of carrier concentrations, Hall mobility and mismatch is probably related to an increase of compensation. Basing on a theoretical calculation of mobility as a function of hole concentration and Vegard's law, we estimate that the compensation comes from the formation of (C-C)⁺_[100] interstitial couples. This fact does not exclude definitively the possibility of the formation of other species such as H-C_As especially for hole concentration lower than 5×10¹⁹ cm⁻³. An annealing under AsH₃+H₂ ameliorates the crystalline properties contrarily to an annealing under N₂. The optical properties have been investigated using Raman spectroscopy. Two main Raman features are observed before and after annealing of the layers: the longitudinal-optic (LO) phonon mode and the coupled plasmon-LO phonon (LOPC). As for as grown layers, the intensity ratio I_LOPC/I_LO between the intensity of LOPC peak and the LO peak increases by increasing the hole concentration. This ratio is about 1 after an annealing of layers under AsH₃+H₂. An unusual change of I_LOPC/I_LO ratio is observed in samples annealed under N₂. Indeed, for high doping (∼10²⁰ cm⁻³) the ratio I_LOPC/I_LO<1 and for relatively low doping (∼2×10¹⁹ cm⁻³) the ratio I_LOPC/I_LO>1. This behaviour is probably related to the high sensibility of Raman measurement not only to the hole concentration change but also to the surface quality.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular beam epitaxial growth of carbon doped GaAs with elemental gallium and arsenic sources and a CCI4 gas source

CC1₄ has been used as a carbon acceptor dopant source for GaAs grown by elemental source molecular beam epitaxy. Deposition of CC1₄ during normal arsenic stabilized growth of GaAs resulted in low mobility, p-type material. Attempts to thermally crack the CC1₄ using a heated gas cracking source resulted in an even lower hole concentration and mobility. One possible explanation for this ineffective acceptor doping behavior, relative to growth environments containing hydrogen (metalorganic chemical vapor deposition) where CC1₄ is an effective dopant, is that hydrogen plays a role in the incorporation of the carbon. Another possible explanation for the poor doping behavior is that the CC1₄ was being modified by the gas cracker, even at relatively low gas cracker temperatures. Further experimentation with different injection schemes will be necessary to better understand the doping behavior. Depositing the CC1₄ onto static, gallium-rich surfaces produces GaAs:C with hole mobilities comparable to GaAs:Be. Average hole concentrations as high as 4 x 10¹⁹ have been demonstrated. Carbon doped AlGaAs/GaAs heterojunction bipolor transistors (HBT_s) have been fabricated with the same characteristics as Be doped HBTs grown in the same MBE system.     

Passivation of carbon acceptors during growth of carbon-doped GaAs,InGaAs, and HBTs by MOCVD

Carbon dopedp-type GaAs and In_0.53Ga_0.47As epitaxial layers have been grown by low-pressure metalorganic chemical vapor deposition using CC1₄ as the carbon source. Low-temperature post-growth annealing resulted in a significant increase in the hole concentration for both GaAs and In_0.53Ga_0.47As, especially at high doping levels. The most heavily doped GaAs sample had a hole concentration of 3.6 × 10²⁰ cm⁻³ after a 5 minute anneal at ≈400° C in N₂, while the hole concentration in In_0.53Ga_0.47As reached 1.6 × 10¹⁹ cm⁻³ after annealing. This annealing behavior is attributed to hydrogen passivation of carbon acceptors. Post-growth cool-down in an AsH₃/H₂ ambient was found to be the most important factor affecting the degree of passivation for single, uncapped GaAs layers. No evidence of passivation is observed in the base region of InGaP/GaAs HBTs grown at ≈625° C. The effect ofn-type cap layers and cool-down sequence on passivation of C-doped InGaAs grown at ≈525° C shows that hydrogen can come from AsH₃, PH₃, or H₂, and can be incorporated during growth and during the post-growth cool-down. In the case of InP/InGaAs HBTs, significant passivation was found to occur in the C-doped base region.     

Self-aligned n-channel GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) using HfO2 and silicon interface passivation layer: Post-metal annealing optimization

In this work, using Si interface passivation layer (IPL), we demonstrate n-MOSFET on p-type GaAs by varying physical-vapor-deposition (PVD) Si IPL thickness, S/D ion implantation condition, and different substrate doping concentration and post-metal annealing (PMA) condition. Using the optimized process, TaN/HfO₂/GaAs n-MOSFETs made on p-GaAs substrates exhibit good electrical characteristics, equivalent oxide thickness (EOT) (∼3.7 nm), frequency dispersion (∼8%) and high maximum mobility (420 cm²/V s) with high temperature PMA (950 °C, 1 min) and good inversion.     

Tellurium-implanted N+ layers in GaAs

Ion implantation of Te was investigated as a doping process for the fabrication of submicron n-type layers in GaAs. The implantation was performed with substrates held at 350°C. After implantation, a protective overcoat of AIN or Si₃N₄ was sputtered on the samples to prevent the GaAs from disassociating during anneal (900°C). The electrical characteristics of the n-type implants were then measured. Current-voltage and capacitance-voltage characteristics of implanted diodes indicated that the junctions were linearly graded and that there was no intrinsic layer present after anneal. Sheet resistivity and Hall effect measurements were used to determine the surface carrier concentration and effective mobility in the implanted layers. Ionized impurity profiles extending beyond the implanted junction depth were calculated by matching differential Hall effect data with junction capacitance-voltage data. A peak electron concentration of 7 × 10¹⁸ electrons/cm³ was observed. However, the profiles exhibited penetrating tails that resulted in junction depths being much deeper than the LSS range theory would predict.     

Efficiency improvement of polymer solar cells by iodine doping

A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C₆₀ butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I₂) doping concentrations. The short circuit current density (J_sc) was increased to 8.7 mA/cm² from 4 mA/cm², meanwhile the open circuit voltage (V_oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage.     

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of high-temperature annealing on GaInP/GaAs HBT structures grown by LP-MOVPE

We have investigated the effect of high-temperature annealing on device performance of GaInP/GaAs HBTs using a wide range of MOVPE growth parameters for the C-doped base layer. Carbon doping was achieved either via TMG and AsH₃ only or by using an extrinsic carbon source. High-temperature annealing causes degradation of carbon-doped GaAs in terms of minority carrier properties even at doping levels of p=1 × 10¹⁹ cm⁻³. The measured reduction in electron lifetime and luminescence intensity correlates with HBT device results. It is shown that the critical temperature where material degradation starts is both a function of doping method and carbon concentration.     

直接键合四结GaAs太阳电池研究

晶圆直接键合技术由于能将表面洁净的两个晶圆集成到一起,从而可以用来制备晶格失配 III-V族多结太阳电池。为了制备GaInP/GaAs/InGaAsP/InGaAs四结太阳电池,需采用具有低电阻率的GaAs/InP键合界面,从而实现GaInP/GaAs和InGaAsP/InGaA上下两个子电池的电学导通。我们设计并研究了具有不同掺杂元素和掺杂浓度的三种键合界面,并采用IV曲线对其电学性质进行表征。此外,对影响键合界面质量的关键工艺过程进行了研究,主要包括表面清洗技术和键合参数优化,例如键合温度、键合压力和键合时间等。最终制备出的键合四结GaInP/GaAs/InGaAsP/InGaAs太阳电池在AM0条件下效率最高达33.2%。     

CdTe/ZnTe/GaAs Heterostructures for Single-Crystal CdTe Solar Cells

Amorphous CdS/single-crystal CdTe solar cells were grown on GaAs substrates by metalorganic chemical vapor deposition. The structures of the films and the electrical properties of the devices were characterized. Highly conducting arsenic-doped ZnTe was grown on GaAs(100) substrates as the buffer layer for CdTe growth. By use of a ～30-nm ZnTe buffer layer, a p-CdTe film with a doping level of ～5×10¹⁶ cm^?3 was achieved. The hole concentration of p-CdTe increased with increasing VI/II ratio under a high As concentration during growth. From temperature-dependent Hall transport measurements, the ionization energy of the As acceptor in the p-CdTe was estimated to be approximately 88 meV. Ohmic behavior of the junctions between CdTe/ZnTe and ZnTe/GaAs was also confirmed. The solar cell performance of this structure, for example an open circuit voltage of 0.63 V, could be improved if the crystal quality of the CdTe film is optimized and the dislocation density of the CdTe film is minimized.     

Efficient dye regeneration in solid-state dye-sensitized solar cells fabricated with melt processed hole conductors

A new method for melting hole transporting materials (HTM) into mesoporous TiO₂ electrodes to obtain solid-state dye-sensitized solar cells (DSSC) is reported. Internal coverage is determined from the efficiency of hole conductor oxidation by photo-oxidized dyes (dye regeneration), measured using transient absorption spectroscopy. High efficiency regeneration indicates complete coverage of the electrode internal surface. A high work function hole conductor (>5.2 eV) was found to give shorter regeneration lifetimes (<1 μs) and better regeneration efficiencies (>90%) than expected. Cell photocurrents were low, but improved after iodine vapor doping of the hole conductor. Counter intuitively, doping also reduced the recombination rate constant 7-fold. A solid state solar cell with power conversion efficiency of 0.075% at 1 sun is reported.     

High-power broad-area InGaNAs/GaAs quantum-well lasers in the 1200 nm range

High-power broad-area InGaNAs/GaAs quantum-well (QW) edge-emitting lasers on GaAs substrates in the 1200 nm range are reported. The epitaxial layers of the InGaNAs/GaAs QW laser wafers were grown on n⁺-GaAs substrates by using metal-organic chemical vapor deposition (MOCVD). The thickness of the InGaNAs/GaAs QW layers is 70 Å/1200 Å. The indium content (x) of the In_xGa_1−xN_yAs_1−y QW layers is estimated to be 0.35-0.36, while the nitrogen content (y) is estimated to be 0.006-0.009. More indium content (In) and nitrogen content (N) in the InGaNAs QW layer enables the laser emission up to 1300 nm range. The epitaxial layer quality, however, is limited by the strain in the grown layer. The devices were made with different ridge widths from 5 to 50 μm. A very low threshold current density (J_th) of 80 A/cm² has been obtained for the 50 μm × 500 μm LD. A number of InGaNAs/GaAs epi-wafers were made into broad-area LDs. A maximum output power of 95 mW was measured for the broad-area InGaNAs/GaAs QW LDs. The variations in the output powers of the broad-area LDs are mainly due to strain-induced defects the InGaNAs QW layers.     

四结叠层太阳电池中AlGaAs/GaAs隧穿结的特性和表现

III-V族化合物叠层太阳电池是具有超高转换效率的第三代新型太阳电池。四结叠层电池GaInP/GaAs/InGaAs/Ge,各子电池的带隙分别为1.8, 1.4, 1.0, 0.7(ev)。为了使各子电池之间电流匹配,在各子电池之间以隧穿结互相连接。本文主要探索研究了四结叠层电池GaInP/GaAs/InGaAs/Ge隧穿结的特性,三个隧穿结的材料选取,探讨了隧穿结对整体叠层电池的特性的补偿作用,对各子电池电流密度的影响,以及在此基础上对整体电池效率的增加。选用AlGaAs/GaAs作为隧穿结运用PC1D进行电池的整体模拟仿真,得到各子电池电流密度分别为16.02mA/cm²,17.12 mA/cm²,17.75 mA/cm²,17.45 mA/cm²,电池在AM0下的开路电压Voc为3.246V,转换效率为33.9%。     

Radiation resistance of MBE-grown GaInP/GaAs-based solar cells

Proton irradiation-based degradation characteristics for molecular beam epitaxy (MBE) grown Ga_0·51In_0·49P/GaAs single-junction tandem solar cells of n/p configuration are reported. The cells were irradiated with 3-MeV protons up to fluences of 10¹³ cm⁻². The cells were characterized with current–voltage (I–V) measurements at AMO conditions, and with spectral measurements. The damage coefficient for the GaAs cells was calculated using numerical modelling by the PC-1D program, and the result was compared with the InP damage coefficient. By using the ‘displacement damage dose’ approach, the degradation characteristics were compared with the published data for InP and GaAs/Ge solar cells. In addition, these MBE results were compared with the radiation behavior of metal-organic chemical vapor deposition (MOCVD)-grown Ga_0·51In_0·49P/GaAs single-, and double-junction solar cells of p/n configuration. © 1998 John Wiley & Sons, Ltd.     

Growth rate reduction in self-limiting growth of doped GaAs by molecular layer epitaxy

Self-limiting growth of GaAs with doping by molecular layer epitaxy (MLE) has been studied using the intermittent supply of TEG, AsH₃, and a dopant precursor, Te(C₂H₅)₂ (diethyl-tellurium: DETe) for n-type growth on GaAs (0 0 1). The self-limiting monolayer growth is applicable at 265°C, however, the growth rate per cycle of doping decreased and saturated at about 0.4 monolayer with increasing doping concentration. To clarify the mechanism of growth with doping and to solve the problem of the growth rate reduction, a doping cycle followed by several cycles of undoped growth was performed. The growth rate reduction in the TEG–AsH₃ system is due to the electrical characteristics of the growing surface, namely, the exchange reaction of TEG is reduced with increasing doping concentration.     

New photoluminescence lines in Vanadium doped GaAs grown by MOVPE

We examined the electrical and optical properties of vanadium-doped GaAs grown by metalorganic vapour phase epitaxy using vanadium tetrachloride (VCl₄) as a novel dopant source. Samples with various vanadium incorporations were investigated. All samples were n type. The electron concentration dependence on the VCl₄ flow rate was established. At 15 K, by comparison with undoped layers grown in the same conditions, photoluminescence spectra of V-doped exhibited three new emission bands: at 1.41, 1 and 0.72 eV. The 1 and 0.72 eV band emissions were attributed to V²⁺ and V³⁺ intracenter emission, while the 1.41 eV band was suggested to be a donor-bound transition. The identity of the donor is tentatively attributed to a donor complex that associates vanadium to an arsenic vacancy. From Hall effect as function of temperature, the donor ionisation energy was estimated to be about 102±5 meV.